Production of sodium carbonate

ABSTRACT

Method of preparing refined, dense soda ash from crude trona by calcining the crude trona to crude sodium carbonate, mixing the crude sodium carbonate with an aqueous solution of soda ash or water to form a substantially saturated crude sodium carbonate solution containing coarse and fine insolubles, clarifying the crude sodium carbonate solution, passing the clarified sodium carbonate solution upwardly through an expanded carbon bed to remove organic impurities, evaporating the carbon-treated sodium carbonate solution to crystallize sodium carbonate monohydrate crystals, separating the sodium carbonate monohydrate crystals and calcining them to dense soda ash.

This invention relates to an improved process for the production ofsodium carbonate (soda ash) from crude trona.

In Sweetwater and adjacent counties in the vicinity of Green River,Wyoming, trona deposits are found at depths ranging from about 800 to1800 feet underground. The main trona bed varies from 8 to 18 feet inthickness, and other beds of less thickness separated by layers of shaleare usually found above the main trona bed. The trona consists chieflyof sodium sesquicarbonate (Na₂ CO₃.sup.. NaHCO₃.sup.. 2H₂ O) and about 4to 12% insoluble materials consisting mainly of shale. A typicalanalysis of the crude trona from which the larger pieces of shale havebeen removed is:

    ______________________________________                                        Constituent            Percent                                                ______________________________________                                        Na.sub.2 CO.sub.3      43.51                                                  NaHCO.sub.3            36.11                                                  H.sub.2 O              13.14                                                  Na.sub.2 SO.sub.4      0.02                                                   NaCl                   0.08                                                   Fe.sub.2 O.sub.3       0.14                                                   Organic Matter         0.30                                                   Insolubles             6.70                                                   ______________________________________                                    

Various processes for the production of sodium carbonate from crudetrona are known. One such process is the sodium carbonate monohydrateprocess. In this process crude trona, after being crushed and screened,is calcined directly to form crude sodium carbonate. This crudecalcinate is dissolved in an aqueous solvent to form a substantiallysaturated solution of sodium carbonate which is then clarified, andsubsequently filtered to remove fine insolubles. The resulting clarifiedsolution is then partially evaporated to crystallize and separate sodiumcarbonate monohydrate crystals which are then calcined to a dense sodaash. This process is described in detail in U.S. Pat. No. 2,962,348,issued to Seglin et al on Nov. 29, 1960. A subsequent modification ofthis process is the installation of carbon columns to treat theclarified sodium carbonate solution with activated carbon in order toremove organic impurities, principally soluble organic compounds, in thefinal product.

One difficulty that has arisen in the operation of the monohydrateprocess, as defined above, has been in the operation of the carboncolumns whose function is to remove organic impurities from theclarified crude sodium carbonate solution. In a typical operation, thecrude sodium carbonate solution is passed through one or more carboncolumns each of which contains a bed of activated carbon particlessuspended on a support, such as a screen or perforated plate. As theclarified sodium carbonate solution passes through the bed, organics areadsorbed on the activated carbon. The carbon treated solution is thenremoved from the carbon bed substantially reduced in organic impurities.Unfortunately, as the crude sodium carbonate solution passes through thecarbon bed, insoluble salts and any unfiltered particles deposit on thecarbon granules. In time, these deposits commence blocking the normalpassages through the carbon bed and increase the pressure drop betweenthe solution entering and leaving the bed at a given rate. The pressuredrop increases with added deposits until they eventually plug the columnentirely and little or no solution can be passed through the bed.

Since the deposition of insolubles which tend to plug the carbon bedoccur principally at the entry point of the clarified sodium carbonatesolution into the bed, it has been the custom to periodically removeportions of the carbon at that end of the column where solution entersthe bed and replace the carbon which is removed with fresh carbon whichis added to the opposite end of the fixed bed. The removed carboncontaining the major proportion of deposits is then treated to removesuch deposits for recovery and recycle of the carbon when possible. Ifthe salts are particularly persistent and cannot be removed by normaltreatment, the encrusted carbon must be discarded.

As a result, it has been desired to carry out the monohydrate process ina manner which will effect a more efficient removal of organicimpurities from the clarified sodium carbonate solution without the needfor constant shutdown to permit removal and replacement of carbon whichhas been encrusted with deposits from the solution being treated.

It has now been found that an improved process for producing dense sodaash from crude trona can be achieved by calcining crude trona to obtaincrude sodium carbonate, mixing the crude sodium carbonate with anaqueous solution of soda ash or water to form a substantially saturatedsolution of crude sodium carbonate containing coarse and fine solids,separating coarse solids from the resulting crude sodium carbonatesolution, clarifying crude sodium carbonate solution, passing theclarified sodium carbonate solution upwardly through an expanded carbonbed and removing organic impurities, evaporating the clarified, carbontreated sodium carbonate solution to crystallize sodium carbonatemonohydrate crystals therefrom, separating the sodium carbonatemonohydrate crystals from their mother liquor and calcining theseparated crystals to produce dense soda ash.

In the drawing, there is illustrated diagrammatically an embodiment forthe production of soda ash from trona by the present process.

In the process of the present invention crude trona is dry-mined andprocessed to crude sodium carbonate by calcining and converting thesodium sesquicarbonate present in the crude trona to sodium carbonate.This reaction may be presented as follows:

    2(Na.sub.2 CO.sub.3.sup.. NaHCO.sub.3.sup.. 2H.sub.2 O)→3Na.sub.2 CO.sub.3 +5H.sub.2 O+CO.sub.2

the crude, dry-mined trona may be prepared for calcination by crushingthe mined trona and passing it over a screening device or other suitableseparating equipment, whereby particles in the general size rangesmaller than one-half inch are collected and passed to the calciner.Rejected oversize particles may then be recycled to the crushingapparatus for further crushing and screening. Proper sizing of the crudetrona insures good conversion of sodium sesquicarbonate therein tosodium carbonate, since oversized particles are not easily converted inthe calciner.

The calcination of the crude trona is necessary in order to convert thesodium bicarbonate values present in the crude trona to sodiumcarbonate. In addition, the crude sodium carbonate resulting from thecalcination has a greater rate of solubility than the crude trona. Afurther result of calcination is that calcium salts and otherdifficultly soluble material in the insoluble fraction are rendered evenless soluble. The increase in the rate of solubility of the crude sodiumcarbonate results in shorter dissolving time and a greater saving in thedissolving equipment size so that large production rates of soda ash canbe obtained in small vessels. Additionally, the shorter dissolving timeresults in less dissolving of calcium carbonate present in the insolublefraction of the trona.

The calcination may be carried out at any temperatures sufficiently highto convert the trona to sodium carbonate, e.g. 150° -800° C. However,since carbon treatment for organic removal is to be employedsubsequently in the process, it is preferred to use lower calcinationtemperatures on the order of 150° to 350° C., and preferably 150° to170° C. Carbon treatment is utilized in a subsequent stage to remove anyorganic matter present in the calcined trona which is carried over intothe process solutions. The retention time of the crude trona in thecalciner is a function of the particle size and the temperature of thecalciner. At a temperature of 150° - 200° C. a period of about 20minutes has been found satisfactory to obtain proper calcination of thecrude trona.

The calcination is usually carried out in a rotary, direct fired kiln,although other types of kilns, such as vertical kilns or grate typecalciners, are equally suitable.

After the crude trona is calcined, it is passed to a dissolving stage(dissolver) where hot water or a hot dilute solution of sodium carbonateis used to form an aqueous solution of the sodium carbonate values insaid calcined crude trona. Any water may be used for dissolution, butpreferably it is softened and contains some dissolved sodium carbonatetherein, on the order of 6% by weight or more. In a preferred operationthe major quantity of liquor used for dissolving the crude soda ash(dissolver influent liquor) is recycled from a subsequent stage of theprocess (thickener stage) and contains sodium carbonate values dissolvedtherein.

The addition of hot, crude sodium carbonate from the calciner to hotdissolver influent liquor may heat the resulting solution above itsboiling point so that steam is formed and escapes from the dissolversused in the dissolving system thereby requiring more water than thattheoretically required for solution of crude sodium carbonate. It ispreferred that the water in the dissolver influent liquor contain nomore than 170 ppm total hardness, expressed as calcium carbonate, sincethis avoids the precipitation of calcium carbonate in the dissolvers.Calcium carbonate precipitation is most undesired because it forms scaleand presents problems in the removal of the scale from the dissolverequipment.

When anhydrous sodium carbonate or calcined crude trona is introducedinto water or into a solution of sodium carbonate in water at atemperature above 95° F. (35° C.) and below 230° F. (110° C.) ithydrates to sodium carbonate monohydrate. If the solution is unsaturatedthe sodium carbonate monohydrate dissolves. If the solution is saturatedthe sodium carbonate monohydrate does not dissolve.

The effluent from the dissolver is a substantially saturated solution ofsodium carbonate plus suspended insolubles such as shales andundissolved sodium carbonate monohydrate, which in our preferredprocess, is then passed to a coarse solids classifier. In the classifierthe coarse solids, on the order of about plus 40 mesh, are separatedfrom the crude sodium carbonate solution and any undissolved orsuspended insolubles smaller than about 40 mesh which may be presenttherein.

It is preferred to utilize as the coarse solids classifier one in whicha conveying means such as a screw is mounted in a liquid-tight elongatedcontainer mounted at an inclined angle from the horizontal, e.g. 10° -60° . The inclined screw is completely covered at its lower end with theslurry to be treated and the upper end of the screw extends beyond andout of the slurry which is present in the classifier. Such a typicalclassifier is the Denver Equipment Company Spiral Classifier Model 125.

In the operation of this classifier, the crude sodium carbonate solutionfrom the dissolver is fed into the intake opening located approximatelyat the center of the classifier. The coarse solids sink to the bottom ofthe classifier because of their heavier weight relative to the finerinsolubles. The inclined screw picks up the heavier particles resting onthe bottom of the classifier and conveys them upwardly out of thesolution towards a discharge chute for the coarse solids located at thetop of the classifier. The fine particles and the remaining sodiumcarbonate solution migrate to the lower end of the classifier and areremoved and passed to a clarifier.

The effluent from the coarse solids classifier is, preferably, asubstantially saturated solution of sodium carbonate containingsuspended insoluble fines, for example no larger than 40 mesh, and ispassed to a clarifier where the fine insolubles (minus 40 mesh) and mudsare permitted to settle out. If a small amount of fine solid matterremains suspended in the liquor after passing the carbonate solutionthrough the clarifier, the liquor may be filtered to remove remaininginsoluble fines.

The overflow liquor from the clarifier, pregnant with sodium carbonate,is then passed upwardly through a carbon column containing an expandedbed of activated carbon. The carbon treatment removes soluble organicimpurities present in the sodium carbonate liquor to prevent theimpurities from being carried through to the final soda ash product.

The term "expanded bed of activated carbon" refers to vertical beds ofcarbon that are supported at their base on a fixed support, such as ascreen or perforated disk, and in which the solution to be purified ispassed upwardly through the bed with sufficient velocity to expand thesize of the bed to at least 5% expansion over its static state withinthe carbon containing column. This requires that sufficient freeboard beavailable above the carbon bed to permit the desired expansion. Itshould be stressed, however, that the velocity used in the bed is not ofsufficient strength to produce what is commonly termed in the art as a"fluidized bed" in which the entire bed is in a fluidized, suspendedstate in which each particle in the bed is supported by the upflowingfluid in a freely moving state. A carbon bed in a fluidized state willnot remove organic impurities efficiently. To illustrate typicalexpanded beds, Table 1 below shows the typical percent of bed expansionobtained at the superficial velocity specified in the Table.

                  TABLE 1                                                         ______________________________________                                        Percent Bed Expansion                                                         ______________________________________                                        Superficial                                                                   Velocity of Bed      Percent Bed                                              (gpm/sq.ft.)         Expansion                                                ______________________________________                                        0.9                   5                                                       2.0                  10                                                       2.9                  15                                                       3.7                  20                                                       4.5                  25                                                       ______________________________________                                    

In this expanded state, the bed of activated carbon is effective inremoving organic impurities from the solution passing through the bed,but any insoluble deposits do not cause pluggage of the column and,therefore, the pressure drop through the column, from the point of entryto the point of exit, does not increase to the point where the carboncolumn must be shut down and the carbon replaced.

The carbon-treated sodium carbonate solution is then passed to anevaporating and crystallizing stage where water is evaporated from thesodium carbonate solution by heating and sodium carbonate monohydrate iscrystallized from the solution. The sodium carbonate monohydrate crystalslurry is then passed to a concentrator where it is treated to removemost of the mother liquor which remains on the crystals. The moistcrystals are then passed to a centrifuge so that essentially all themother liquor is removed. Thereafter the crystals are heated in a dryerat a temperature of 105° to 125° C. to convert the sodium carbonatemonohydrate crystals to anhydrous soda ash. Dehydration of the sodiumcarbonate monohydrate permits recovery of a dense form of soda ashhaving a bulk density on the order of about 60 pounds per cubic foot.

Referring now to the drawing; the drawing illustrates diagrammaticallyan embodiment of the invention for production of soda ash from calcinedtrona.

In the drawing the crude trona, crushed to a general size range of lessthan one-half inch is fed by conduit 10 to a calciner 12 where the crudetrona is converted to crude sodium carbonate by heating at elevatedtemperatures, preferably at temperatures on the order of 150° to 350°C., more preferably at 150° to 170° C. The calciner 12 illustrated inthe drawing is a direct fired calciner, although other known calcinerscan also be employed, and the gaseous products of reaction, namely waterand CO₂, are removed from the calciner and vented by means not shown.

The crude sodium carbonate recovered from calciner 12, with or withoutcooling, is passed through line 14 to dissolvers 16 where the sodiumcarbonate is dissolved in a hot dissolver influent liquor suppliedthrough line 18 to form a substantially saturated solution of the sodiumcarbonate. The influent liquor may be water, preferably softened watersupplied through line 20, and more preferably recycled solution suppliedthrough line 18, from the thickener stage 36 of the process. In thepreferred embodiment of the invention the dissolver influent liquor usedfor dissolving sodium carbonate contains at least 6% of sodium carbonatedissolved therein such that the total hardness of the liquor is lessthan 170 ppm total hardness, calculated as calcium carbonate. Thisliquor is normally supplied through line 18 from the thickener 36 butcan also contain some makeup water through line 20, preferably softenedmakeup water. Dissolution of the sodium carbonate in the dissolvernormally takes place with a hot solution at temperatures on the order of85° to 95° C.

The crude sodium carbonate solution and suspended solids are removedthrough line 22 and passed into a coarse solids classifier 24. Inclassifier 24 the coarse solids, that is solids of at least about 40mesh, fall to the bottom of the classifier and are passed upwardly bymeans of the revolving screw in the classifier. The sodium carbonatesolution and the finer particles, that is particles having a sizesmaller than about 40 mesh, flow towards the base of the classifier andare removed through line 28. As the coarse solids are removed from theliquid layer present in classifier 24 the solids are sprayed by means ofspray heads 22A with a washing liquid which may be water or a dilutesodium carbonate solution, to free the solids from saturated sodiumcarbonate solution which is retained on the surface. The washing liquidafter contacting the solids now contains increased sodium carbonatevalues and is mixed with the sodIum carbonate solution at the base ofthe classifier 24. The washed solids removed through line 26 are sentfor disposal. The sodium carbonate solution, containing only insolublefines is removed through line 28 and passed to clarifier 30.

In clarifier 30 the sodium carbonate solution, free of coarse solids,remains in a substantially quiescent stage. The insoluble fines settleand a sludge forms at the bottom of the clarifier which is removed by arotating rake and passed to a thickener through line 32. Makeup water tothe process may be added through line 34 and mixed with the muds fromthe classifier 30 in thickener 36 to soften the makeup water. Theinsoluble fines and muds settle in thickener 36 and are removed by line38 to waste. Water which has been softened and which contains sodiumcarbonate values, preferably above 6% by weight, is then removed fromthe thickener and is passed through line 18 to dissolver 16 for use indissolving crude sodium carbonate.

The relatively clear solution that overflows from the clarifier 30through line 40 is passed to the filter 42 to remove any suspendedsolids remaining in the liquor. The clarified sodium carbonate solutionis then passed through line 43 into an expanded carbon column 44 toremove soluble organic impurities. The carbon-treated sodium carbonatesolution is then passed through line 46 into a polish filter 48 toremove any additional fines or carbon particles.

The purified sodium carbonate solution is passed through line 50 into anevaporator 58. The sodium carbonate solution enters a recycle circuit 56which flows into a heat exchanger 52, for heating solution passingthrough this exchanger. The heat exchanger 52 is supplied with steam 54in order to supply heat to the solution circulating in the recyclestream 56. The heated solution is then passed into the evaporator 58where a portion of the water is evaporated and is removed through line60 and condensed in condenser 62. As a result of the evaporation, sodiumcarbonate monohydrate is crystallized and a crystal slurry is removedthrough line 66. The condensed water 64 removed from condenser 62 can bepiped to cooling towers or can be used as softened makeup water, asdesired.

The sodium carbonate monohydrate crystal slurry removed through line 66is passed into concentrator 68 to remove the bulk of the mother liquorremaining on the sodium carbonate monohydrate crystals. A moreconcentrated crystal slurry is removed from concentrator 68 through line70 and is then passed into a centrifuge 72 where very small amounts, onthe order of 3% or so of mother liquor, remain on the sodium carbonatemonohydrate crystals. The mother liquor which is removed fromconcentrator 68 and centrifuge 72 are combined, and recycled throughline 74 to the purified sodium carbonate solution 50 which is fed to theevaporator. The centrifuged sodium carbonate monohydrate crystals areremoved from centrifuge 72 through line 76 and passed into a dryer 78where the sodium carbonate monohydrate crystals are converted to sodiumcarbonate and removed through line 80 as product. The dryer 78 is heatedat a temperature sufficient to convert the sodium carbonate monohydrateto soda ash, for example 105° to 125° C., and may be in the form of afluid-bed dryer, rotary kiln dryer or the like. As a result ofconverting the sodium carbonate monohydrate to soda ash the resultingsoda ash product is recovered as a high density material having a bulkdensity of about 60 pounds per cubic foot.

The following example is given to illustrate the invention and is notintended to be limiting thereof.

EXAMPLE Run A - Process of the Invention

The process employed was that set forth in the drawing in which aclarified solution of sodium carbonate was passed upwardly through anexpanded bed of activated carbon at a rate of 2.11 gallons per minuteper square foot of bed for a period of 30 days. The initial pressuredrop across the column was 26 psig. At the end of 30 days of operation,the pressure drop was still below 40 psig.

Run B - Prior Art

The above was repeated except that the clarified sodium carbonatesolution was run through a packed carbon column at a rate of 3.16gallons per minute per square foot of bed. However, the bed could not beexpanded because it was held in place by screens and had no freeboardabove the bed. The pressure drop across the bed of carbon is set forthbelow.

    ______________________________________                                        Days of Operation  Pressure drop, psig                                        ______________________________________                                        1                   67                                                        2                   71                                                        8                  104                                                        9                  107                                                        16                 146                                                        ______________________________________                                    

As will be observed from the above examples, the prior art method (RunB) after operating for 8 days resulted in a pressure drop that wasgreater than that obtained after 30 days operation with the method ofthe present invention.

Pursuant to the requirements of the patent statutes, the principle ofthis invention has been explained and exemplified in a manner so that itcan be readily practiced by those skilled in the art, suchexemplification including what is considered to represent the bestembodiment of the invention. However, it should be clearly understoodthat, within the scope of the appended claims, the invention may bepracticed by those skilled in the art, and having the benefit of thisdisclosure, otherwise than as specifically described and exemplifiedherein.

What is claimed is:
 1. Process for preparing dense sodium carbonate from trona which comprises calcining crude trona to obtain crude sodium carbonate, mixing the crude sodium carbonate with an aqueous liquor to form a solution of crude sodium carbonate containing coarse and fine solids, clarifying the crude sodium carbonate solution, passing the clarified sodium carbonate solution upwardly through an activated carbon bed at a flow velocity sufficient to provide at least a 5% expansion of said bed over its static state, but wherein said velocity is insufficient to form a fluidized bed, to remove organic impurities, evaporating the clarified carbon-treated sodium carbonate solution to crystallize sodium carbonate monohydrate crystals therefrom, separating said sodium carbonate monohydrate crystals from their mother liquor and calcining the separated crystals to produce dense soda ash.
 2. Process of claim 1 wherein the crude trona is calcined at temperatures of from about 150° to 350° C.
 3. Process of claim 2 wherein said expanded bed of activated carbon has 5% to 25% expansion over its static state.
 4. Process of claim 1 wherein said clarified sodium carbonate solution is passed through said expanded bed of activated carbon at a rate of from about 0.9 to 4.5 gallons per minute per square foot of bed. 